This study was conducted to determine the effects of dietary cellulase addition on improving the nutritive value of Chlorella for juvenile crucian carp Carassius auratus (initial body weight: 2.99 +/- 0.02 g, mean +/- SEM). Five isonitrogenous and isoenergetic experimental diets were formulated to contain 0.0 (control), 0.5, 1.0, 1.5 and 2.0 g kg(-1) cellulase, respectively. Each experimental diet was randomly assigned to triplicate groups with 25 juvenile fish per fibreglass tank for 8 weeks. The results showed that weight gain, specific growth rate, feed intake and the trypsin activity in the anterior intestine increased with increasing dietary cellulase to 1.5 g kg(-1) and then declined with further addition. However, the mRNA expression levels of Mrf4 and Myf5, the apparent digestibility coefficients for dry matter, protein, energy and the majority of amino acids, and the activity of lipase in the anterior intestine were highest in fish fed the 1.0 g kg(-1) cellulase diet, and then tended to decline with further cellulase supplementation. In conclusion, the optimal dietary cellulase supplementation level was 1.0-1.5 g kg(-1), which can improve growth performance, digestive activities and nutrient digestibility in crucian carp.